It would be beneficial for different industries including those in the defense, law enforcement, environmental, food, medical, and materials fields to be able to detect trace amounts of gas-phase analytes using a reliable spectroscopic technique. Such a technique would allow them to detect contraband such as drugs, explosives, and/or contaminants on site. Unfortunately, however, very few spectroscopic techniques are sensitive enough to detect trace amounts of gas-phase analytes.
Intracavity laser absorption spectroscopy or “ICLAS” is one of the few spectroscopic techniques capable of doing so. In ICLAS, a test substance is introduced into the cavity of a laser that oscillates simultaneously across multiple resonator modes. If the test sample contains a substance that absorbs in the wavelength range emitted by the laser, the absorption features affect the laser spectrum by a measureable amount. ICLAS is very sensitive because it allows for extremely long effective path lengths and high spectral resolution.
Many molecules have a characteristic vibrational and/or rotational absorption spectrum in a particular band of the infrared wavelength region. This band, which ranges from wavelengths of about 3 μm to about 12 μm, is known as the “molecular fingerprint region” because the fundamental rotational/vibrational absorption bands for most molecules fall within these wavelengths. Because each molecule exhibits a unique absorption spectrum in the fingerprint region, it is often used to qualitatively identify molecules.
Quantum cascade lasers or “QCLs” are promising laser sources for performing ICLAS in the infrared wavelength region because they have broad gain spectra, a wide range of wavelengths, high output power, high duty cycle, and the ability operate at room temperature. The fingerprint region is easily accessible with QCLs. Combining a QCL with the ICLAS technique allows one to obtain the highest possible absorption cross-section because of the long path lengths and wavelengths that may be employed.
The inventor and his co-workers have already demonstrated that molecular detection using QCLs in an external cavity is possible. This previous work is described in the following references: (1) Medhi, et al. “Infrared Intracavity Laser Absorption Spectrometer,” Proc. Intl. Symp. Spectral Sensing Research (ISSSR), June 2010; (2) Medhi, et al., “Infrared Intracavity Laser Absorption Spectrometer,” Next Generation Spectroscopic Technologies III, Proceedings of SPIE, Vol. 7680, Apr. 21, 2010; (3) Medhi, et al., “Sensitivity of long-wave infrared intracavity laser absorption vapor detector,” Laser Resonators, Microresonators, and Beam Control XIV, Vol. 8236, Jan. 21, 2012; and (4) Medhi, et al., “Intracavity laser absorption spectroscopy using mid-IR quantum cascade laser,” Next Generation Spectroscopic Technologies IV, Proceedings of SPIE, Vol. 8032, May 12, 2011. Each of these references is incorporated by reference herein in its entirety.
These references describe the development of a highly sensitive external cavity QCL-based ICLAS sensor by coupling an external cavity QCL with a Fabry-Perot interferometer. The Fabry-Perot interferometer included a pair of mirrors spaced apart to form a Fabry-Perot resonator. In order to record a spectrum, the spacing between the mirrors was adjusted so that the wavelength of the laser beam corresponded to a resonance mode of the Fabry-Perot resonator. This technique was shown to be very sensitive to trace gases but involves some serious drawbacks, which are discussed below.